Marijuana decriminalization is a hotly debated topic, which has nonetheless seen popular support in recent years. Current federal law (the Controlled Substances Act) conflicts with many state decriminalization efforts, raising the obvious question of federal preemption. The Supreme Court has failed to provide a clear answer on how much federal law preempts state marijuana decriminalization laws. This Article identifies the foundational principles of vertical and horizontal preemption, as well as various unanswered questions regarding these doctrines. It then applies these questions to marijuana decriminalization. Ultimately, it argues that there is a weak case for vertical or horizontal preemption in the marijuana decriminalization context.